Methods and compositions are described for treating water- and hydrocarbon-producing subterranean formations with a relative permeability modifier at concentrations below those conventionally used in the art, while not substantially impacting the treatment fluid's performance. Treatment fluids described herein comprise a base fluid, a relative permeability modifier comprising a hydrophobically modified hydrophilic polymer, and a companion polymer interacting synergistically with the relative permeability modifier such that the treatment fluid is operable to reduce the water permeability or to increase an aqueous fluid injection pressure of at least a portion of a subterranean formation by more than the relative permeability modifier or the companion polymer acting alone at like concentration. The water permeability reduction and/or the aqueous fluid injection pressure increase can be more than additive. Methods described herein comprise placing such treatment fluids in at least a portion of a subterranean formation.